In contrast to conventional surgery, which requires a relatively large incision in order to gain access to a surgical site within a body, endoscopic procedures utilize natural passages, or, alternatively, involve the formation of very small portals to gain access to the surgical site of interest. Accordingly, an endoscopic procedure is often referred to as minimally invasive surgery. One advantage of performing endoscopic surgery is that since the portions of the body that are cut are reduced, the portions of the body that need to heal after the surgery are likewise reduced. Still another advantage of endoscopic surgery is that it exposes less of the interior tissue of the body to the open environment. This minimal opening of the body lessens the extent to which its internal tissue and organs are open to infection.
Advancements in this field of “closed” surgery, such as arthroscopy and, more generally, endoscopic surgery, have led to the creation of numerous minimally invasive surgical cutting instruments. As noted above, in closed surgery, access to the surgical site is gained via one or more portals. As such, the instruments used in the surgical procedure must be sufficiently smooth and elongated to permit the distal ends of the instruments to reach the surgical site with minimal trauma to neighboring tissues. One end of the instrument, often referred to as the “distal end”, is designed to be positioned at the surgical site. The opposed end of the instrument, often referred to as the “proximal end”, extends out of the patient's body. The distal end of the instrument is typically provided with some type of working head designed to manipulate the tissue against which it is placed whereas the proximal end of the instrument is provided with a mechanism for the user to remotely control the working head.
Surgical cutting instruments for use in closed surgery—often referred as endoscopic “shavers”—are typically composed of a pair of concentrically disposed, close-ended, generally tubular members, more typically an elongated outer tubular member terminating in a distal opening or “cutting window”, an aperture situated in the distal end, on the distal end side wall, or both, and an elongated inner tubular member, slidably and concentrically disposed in the outer tubular member, whose distal end is disposed adjacent the cutting window of the outer tubular member. The distal end of the inner tubular member typically has a surface or edge for engaging tissue via the distal opening in the outer tubular member and cooperates with the opening to shear, cut or trim tissue, a process often referred to as “resection”. For example, the inner tubular member may be rotatably driven about its axis from its proximal end by a handpiece having a small electric motor which is controlled by one or more finger actuated switches on the handpiece, one or more foot switches on a console supplying power to the handpiece, or some other analogous control means. Cut tissue can then be aspirated through the hollow lumen of the inner tubular member to be collected via a vacuum tube communicating with the handpiece. The distal end of the inner tubular member can be provided with a number of dimensions or configurations, depending upon the surgical procedure to be performed. Similarly, the opening in the distal end of the outer tubular member may be adapted to cooperate with the particular configuration of the distal end of the inner tubular member. For example, the inner and outer tubular members can be configured to produce side cutting or end cutting, or a combination of the two, to cut soft or bony tissues or combinations thereof. These various configurations are generally referred to in the art as “shaver blades”.
The cutting windows of a shaver each have perimeters composed of two relatively longitudinal, straight or curvilinear edges connected at their proximal and distal ends by two relatively transverse edges. The configuration of the longitudinal edges, and to a lesser extent the transverse edges is determined by the intended use of the shaver. For instance, shavers intended for use on soft tissue will be provided with cutting windows configured for increased resection efficiency but relatively low resistance to deformation since the cutting forces are typically low. Conversely, those shavers intended for use on tough tissue, such as meniscus or vertebral discs, will be provided with a greater resistance to deformation since the cutting forces are quite high.
Resection of tissue by a shaver blade is typically accomplished by the cooperative interaction between the sharp beveled edges of the inner and outer cutting windows. As the inner and outer windows come into alignment, vacuum within the lumen of the inner member aspirates tissue into the opening formed. Continued rotation of the inner member causes the inner cutting edges to approach the outer cutting edges. Tissue in the cutting window between the inner and outer edges is either trapped between the edges or ejected from the window. Tissue trapped between the edges is either cut by the edges as they approach each other or torn by the cutting edges as they pass and rotate away from each other. The resected tissue is then aspirated from the site through the inner lumen of the inner member.
Resection efficiency can be improved by decreasing the relative portion of the material that is ejected from the window, and increasing the portion that is trapped between the edges and resected. Decreasing the relative portion ejected from the window can be achieved through the use of sharper cutting edges. Illustrative means for increasing the sharpness of the cutting edge include decreasing the included angle of the cutting edge, decreasing the edge radius, and decreasing the roughness of the surfaces over which tissue must slide during resection. For example, U.S. Pat. No. 5,843,106 (Heisler) discloses a shaver with increased resection efficiency produced by an outer cutting window configuration having “sharpened” low included-angle cutting edges. The relative portion of tissue ejected from the window during closure may also be decreased by adding teeth to either the inner cutting edges or outer cutting edges or both. Shavers having inner cutting edges with teeth are described in the art, for example in U.S. Pat. Nos. 5,217,479 (Shuler) and 5,269,798 (Winkler), each of which disclose shavers having inner cutting edges with teeth, such teeth being formed by a “through-cutting” process, such as wire electrical discharge machining (wire EDM), or by grinding. Teeth so formed are efficient at retaining tissue within the window so that it can be cut by the low included angle outer cutting edges as the inner and outer edges converge. The inner cutting edges do little cutting since the teeth form a very large included angle cutting edge.
The Cuda™ by Linvatec Corporation (Largo, Fla.) and the Tomcat™ by Stryker Corporation (Kalamazoo, Mich.) each have teeth on both the inner and outer cutting edges, the edges being formed by a two-dimensional, through-cutting process such as grinding or wire EDM. The edges formed have large included angles, a geometry that is inefficient for cutting tissue. Shavers having these two-dimensionally shaped teeth on the inner and outer cutting edges separate tissue principally by tearing as the edges pass each other when the cutting window is closed. Such tearing is undesirable since the torn tissue frequently becomes trapped in the gap between the inner and outer tubular members, thereby causing clogging. This problem is specifically addressed in U.S. Pat. No. 6,053,928 (Van Wyk et al.), which discloses a shaver having a plurality of teeth on the laterally opposed cutting edges of an outer window, the cutting edges being symmetrical when viewed in a plane normal to the axis of the tube. The cutting edges are formed so that, when viewed in any such plane, the edges have low included angles, in the valleys between the teeth as well as the teeth. The Great White™ shaver by Linvatec, constructed in accordance with the principles of the '928 patent, is very efficient at resecting tissue and experiences reduced clogging due to the sharpness of the outer cutting edges.
When a shaver is used with a constant rotation imparted to the inner member, tissue in close proximity to the window is sucked into the window and either resected or ejected from the window in the manner previously herein described. Tissue which is ejected from the window, or the remaining tissue adjacent to a resected portion, is swept in the direction of the rotation. When the cutting window is opened again by the rotation of the inner member, the amount of tissue which will be pulled into the window by vacuum in the inner lumen is diminished from that of the previous opening event because of this directional “set” of the tissue. That is, because the tissue is already preferentially oriented in the direction of the rotation of the approaching inner cutting edge, it is difficult for that inner cutting edge to get sufficient “bite” to retain the tissue in the cutting window for resection. Because of this, arthroscopic shavers are generally used in an “oscillate” mode when cutting tissue. In this mode the inner is rotated in one direction for a predetermined number of revolutions, whereupon its rotation is reversed for the same predetermined number of revolutions. The inner cutting edges approach the tissue from alternating directions thereby greatly increasing the relative portion of tissue that is sucked into the window and is resected rather than ejected.
As noted above, a conventional shaver blade assembly is composed of a stationary outer assembly and an inner assembly. The inner assembly is typically composed of a generally tubular member with a closed distal end and a proximal-end hub configured for removable coupling a drive mechanism of a powered handpiece so as to transmit rotational motion from the handpiece to the distal end of the inner assembly. The outer assembly is typically composed of a generally tubular member with a closed distal end and a proximal hub means for removably mounting the shaver blade assembly in a powered handpiece. An elastic member transmits an axial force distally on the inner assembly so that contact is maintained between the outer surface of the distal end of the inner member and the inner surface of the distal end of the outer member, the surfaces together functioning as a bearing. In some shaver systems, the elastic member is a spring affixed to the hub of the inner assembly. In other systems, the elastic member is a spring in the handpiece in which the shaver is mounted. The distal bearing surfaces are spherical on most (almost all) shavers, although shavers with other shapes are produced for specialized purposes. The radius of the spherical inner surface of the outer member is slightly larger than that of the spherical external surface of the inner member. The application of the axial force to the inner member by the elastic member creates quite high Herzian contact stresses at the bearing surfaces. Since shavers are used with rotational speeds as high as 5,000 rpm, chafing or galling of these surfaces is frequently a problem. To prevent galling, the materials of the inner and outer distal ends are carefully selected and the components hardened and machined to very precise shapes, frequently with form tolerances of as little 0.0002 inches. The surface finishes of the bearing surfaces are also critical since irregularities in the surfaces can lead to high localized stresses which result in galling of the surfaces during use. Galling of the bearing surfaces during use creates metallic debris which can be deposited into the surgical site, with negative consequences to the patient. In severe cases, galling may cause welding of the inner and outer members so as to make the shaver unusable. As a result, some manufacturers coat the inner member bearing surface with a gall-resistant metallic material, while others make the distal end of the inner member from a gall resistant alloy. In any event, galling and metallic debris created by shaver blades is still a frequent problem since inspection of the inner surface of the outer member is very difficult and minor manufacturing abnormalities can create surfaces which are not to specification.
Closely related to arthroscopic shavers are a category of devices known in the art as arthroscopic burs, which are used for resecting bone. Burs differ from shavers in that the inner member has multiple cutting edges arranged on a rotating element (the bur head), with cutting achieved solely by the inner cutting edges. While shavers cut by cooperative interaction of the inner and outer cutting edges, burs cut with the inner edges only. Also, shavers use a vacuum to draw tissue into a cutting window for resection while burs use suction only to remove debris from the surgical field. Burrs are ineffective for cutting soft tissue. Examples of commercially available burrs include, but are not limited to, the Spherical Burr, Oval Burr, Cyclone Burr, and Vortex Router by Conmed Corporation (Utica, N.Y.).
The Helicut™ burr by Smith and Nephew Incorporated (Andover, Mass.), and the Lightning™ by Conmed Corporation are specialty burrs which cut both soft tissue and bone. The instruments have a helical rotational inner member with two cutting edges, and an open-ended outer member with edges which cooperatively resect soft tissue with the edges of the inner member. The resected tissue is removed from the site by the action of the helical inner as well as by a vacuum applied to the proximal end of the outer member. The Helicut and Lightning are unique in that they resect both bone and soft tissue, and does not have a tubular inner member. Because the outer tube does not have a closed end, there is no bearing surface at the distal end, the bearing surface being between the inner and outer hubs at the proximal end of the device.
Thus, there are a number of commercially available embodiments of endoscopic cutting instruments. Nevertheless, despite the above described improvements, there remains a clear need in the art to increase the efficiency of endoscopic cutting instruments and shaver blades. The present invention is directed to such a need.